[lldb] Create dependent modules in parallel (llvm#114507)

Create dependent modules in parallel in Target::SetExecutableModule.
This change was inspired by llvm#110646 which takes the same approach when
attaching. Jason suggested we could use the same approach when you
create a target in LLDB.

I used Slack for benchmarking, which loads 902 images.

```
Benchmark 1: ./bin/lldb /Applications/Slack.app/Contents/MacOS/Slack
  Time (mean ± σ):      1.225 s ±  0.003 s    [User: 3.977 s, System: 1.521 s]
  Range (min … max):    1.220 s …  1.229 s    10 runs

Benchmark 2: ./bin/lldb /Applications/Slack.app/Contents/MacOS/Slack
  Time (mean ± σ):      3.253 s ±  0.037 s    [User: 3.013 s, System: 0.248 s]
  Range (min … max):    3.211 s …  3.310 s    10 runs
```

We see about a 2x speedup, which matches what Jason saw for the attach
scenario. I also ran this under TSan to confirm this doesn't introduce
any races or deadlocks.

lldb/source/Target/Target.cpp

@@ -68,6 +68,7 @@
 
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/Support/ThreadPool.h"
 
 #include <memory>
 #include <mutex>
@@ -1575,7 +1576,6 @@ void Target::SetExecutableModule(ModuleSP &executable_sp,
                m_arch.GetSpec().GetTriple().getTriple());
     }
 
-    FileSpecList dependent_files;
     ObjectFile *executable_objfile = executable_sp->GetObjectFile();
     bool load_dependents = true;
     switch (load_dependent_files) {
@@ -1591,10 +1591,14 @@ void Target::SetExecutableModule(ModuleSP &executable_sp,
     }
 
     if (executable_objfile && load_dependents) {
+      // FileSpecList is not thread safe and needs to be synchronized.
+      FileSpecList dependent_files;
+      std::mutex dependent_files_mutex;
+
+      // ModuleList is thread safe.
       ModuleList added_modules;
-      executable_objfile->GetDependentModules(dependent_files);
-      for (uint32_t i = 0; i < dependent_files.GetSize(); i++) {
-        FileSpec dependent_file_spec(dependent_files.GetFileSpecAtIndex(i));
+
+      auto GetDependentModules = [&](FileSpec dependent_file_spec) {
         FileSpec platform_dependent_file_spec;
         if (m_platform_sp)
           m_platform_sp->GetFileWithUUID(dependent_file_spec, nullptr,
@@ -1608,9 +1612,48 @@ void Target::SetExecutableModule(ModuleSP &executable_sp,
         if (image_module_sp) {
           added_modules.AppendIfNeeded(image_module_sp, false);
           ObjectFile *objfile = image_module_sp->GetObjectFile();
-          if (objfile)
-            objfile->GetDependentModules(dependent_files);
+          if (objfile) {
+            // Create a local copy of the dependent file list so we don't have
+            // to lock for the whole duration of GetDependentModules.
+            FileSpecList dependent_files_copy;
+            {
+              std::lock_guard<std::mutex> guard(dependent_files_mutex);
+              dependent_files_copy = dependent_files;
+            }
+
+            // Remember the size of the local copy so we can append only the
+            // modules that have been added by GetDependentModules.
+            const size_t previous_dependent_files =
+                dependent_files_copy.GetSize();
+
+            objfile->GetDependentModules(dependent_files_copy);
+
+            {
+              std::lock_guard<std::mutex> guard(dependent_files_mutex);
+              for (size_t i = previous_dependent_files;
+                   i < dependent_files_copy.GetSize(); ++i)
+                dependent_files.AppendIfUnique(
+                    dependent_files_copy.GetFileSpecAtIndex(i));
+            }
+          }
+        }
+      };
+
+      executable_objfile->GetDependentModules(dependent_files);
+
+      llvm::ThreadPoolTaskGroup task_group(Debugger::GetThreadPool());
+      for (uint32_t i = 0; i < dependent_files.GetSize(); i++) {
+        // Process all currently known dependencies in parallel in the innermost
+        // loop. This may create newly discovered dependencies to be appended to
+        // dependent_files. We'll deal with these files during the next
+        // iteration of the outermost loop.
+        {
+          std::lock_guard<std::mutex> guard(dependent_files_mutex);
+          for (; i < dependent_files.GetSize(); i++)
+            task_group.async(GetDependentModules,
+                             dependent_files.GetFileSpecAtIndex(i));
         }
+        task_group.wait();
       }
       ModulesDidLoad(added_modules);
     }